Where are you located? If you are in New England, my first recommendation would be Poulin Grain's E-Tec. If you're not, do you have Blue Seal feeds available in your area? Blue Seal has a Lifetime formula in a gray bag that I also really like.
Other than grain, I would say beet pulp also. I can't say it enough times. Beet pulp is fantastic for putting on weight, keeping on weight, its great for digestion and horses like it.
Have her teeth checked as soon as possible, and until you have that done, adding beet pulp will help, as will soaking her grain. In the event it IS her teeth, it will make eating a lot easier and it will help with the breakdown of the grain if her teeth aren't doing the job well. If it's not her teeth, no big deal. When you get the okay on her teeth- stop soaking her grain.
That's what I would do anyway.
Chances are she will start to put on weight anyway since you will be caring for her properly.